Understanding language is crucial to understanding mental retardation because language controls, express, or mediates much of our three major developmental spheres: the intellectual, social, and physical realms. For this reason, language must be a central focus of understanding and research in mental retardation. The proposed research on language was guided by a conceptualization of human development that stress the centrality of the developmental cusp, a change, whether seen as one of behavior, skill, ability, perception, or motivation, that is crucial to what should come next in normal development. A cusp is a change that, if not made, means that little further development is possible or likely in its realm but, once made, permits a significant set of subsequent developments to occur. The cusps chosen to be studied in the proposed research were ones that appear to be critical to the process of language and communicative development, are representative of a diversity of points in communicative development (from early and rudimentary to late and sophisticated), are representative of the problems of a diversity of people including those with mental retardation, span both the intellectual and social realms of development, and seem amenable to realistic intervention so as to produce findings that will contribute directly to preventing or remediating some aspects of mental retardation. The cusps proposed to be studied are: (1) The emergence of the use of words and gestures that signify action or events at a distance among children with mental retardation and the emergence of symbolic referential communication; (2) The rate at which typically developing and children with Down syndrome add new words to their expressive vocabulary in their natural homes; (3) The development of the ability to classify or categorize objects and events together in simple and more complex ways among children with mental retardation; (4) The emergence of the mediating function of stimulus names in the development of equivalence concepts among people with mental retardation; (5) The development of the mediating function of self-instructions in problem solving among typically developing and children with mental retardation; (6) The development of positive relationships that contribute to more effective and appropriate interactions between adults with mental retardation and the people who teach them.